The field of the invention comprises fiber optic sensors and, in particular, intensity-modulated fiber optic sensors such as microbend and diaphragm type sensors. These devices operate on the principle of relating the changes in light intensity transmitted by an optical fiber to the changes in a measured parameter such as pressure.
In a microbend sensor, a measured parameter alters light intensity by exerting stress or microbend deformations on a section of optical fiber. Such a sensor is disclosed in U.S. Pat. No. 5,421,195 to one of the applicants. In a diaphragm sensor, the light reflected by a flexing diaphragm changes proportionally with the measured parameter. Such a sensor is disclosed in U.S. Pat. No. 5,390,546 also to one of the applicants.
While intensity modulated sensors can be robust and of low cost, they may be affected by environmental and handling factors which in turn will result in a sensor error. These undesirable factors may include fiber bending, optical connector thermal and mechanical instabilities, optical throughput changes due to exposure of optical fibers to high temperatures, changes in optical coupling efficiency between optical sources (e.g., light emitting diodes (LED)), detectors (e.g., PIN photodiodes) and an optical fiber, separate temperature effects on LEDs and photodiodes, or aging of these components.
Compensation techniques and devices have been reported in the past to reduce the susceptibility of intensity-modulated sensors to environmental and handling errors. Prior reports describe dual wavelength, multi-fiber, and time of flight compensation techniques for both microbend and diaphragm-type sensors.
The prior techniques and devices tend to be expensive and complex and therefore not well suited for low-cost sensors aimed at industrial control applications.